A treatment proposal is a set of estimates about your water, written up as a price. Before you compare totals, find out which water numbers each provider used to make those estimates. Two proposals can look similar on paper and still start from different facts, and the cheaper one is only a bargain if its starting facts are right. For county-wide context, the St. Charles Water Ratings home page summarizes treatment information for the area, but your own address is what the equipment has to match.
Ask for the water data the design rests on
Ask each provider to name the test results they relied on. A solid answer points to a specific lab report that lists the sample source (a well, a municipal tap, or a particular faucet), how recent the sample is, and which tests were run. If the answer is 'we checked your water' or a verbal estimate, ask them to write the numbers down. You are not asking for their opinion of your water. You are asking for the inputs that the design depends on.
Check the numbers that drive size and cost
Ask which of these readings set the equipment size and the media or resin choice: hardness, iron and manganese, pH, chlorine or chloramine, and sediment. Where a health-related result is involved, such as bacteria or nitrate, ask whether it was tested at all and whether the proposal treats it or only mentions it. Then ask what daily flow rate and peak demand the design assumes, and how many people and fixtures that reflects. Finally, ask whether the figures come from your own test or from a utility report, and whether that report is current for your address.
Send every provider the same inputs
Once you have the numbers, send the same figures to every provider and ask each one to confirm or correct them in writing. If one proposal lists a larger unit, a different media, or a different regeneration or change schedule, ask which input caused the difference. Sometimes the reason is sound, such as a higher iron reading or a heavier daily use pattern. Sometimes the quote is simply padded. Either way, you learn what you are paying for, and you can compare the proposals on the same footing.
Check what the proposal does not treat
A proposal should state what the equipment is designed to address and what it is not. If the water has a result the unit does not target, the paper should say so and should not imply that the unit handles it. Ask for a plain sentence on that point. If your existing testing does not cover a concern you have, ask the provider or your water utility which test would answer it and who runs that test. Those answers belong in the file before you sign.
Keep the basis with the signed quote
File the lab results, the design inputs, and the signed proposal together. When the equipment is serviced later, the first question is always what it was sized for. If that answer is in the file, the service visit goes faster, and the next comparison starts from facts you already have. For a closer look at how the county grades are assembled, see How Grades Work: Criteria, Weights, and the A to F Scale.